Find PID on other end of pipe

darwin.sh

#!/bin/bash

pid=$$
pgid=$(ps -o pgid= -p $pid)

# list files opened by process group
lsof=$(bash -c 'echo $$; exec lsof -g '"$pgid")

# get pid of lsof
lpid=$(echo "$lsof" | head -1)

# filter for pipes, exclude files opened by lsof
lsof=$(echo "$lsof" | awk '$6=="PIPE" && $2!="'"$lpid"'" {print}')

# find own stdin device
pipe=$(echo "$lsof" | awk '$5=="0" && $2=="'"$pid"'" {print substr($9,3)}')

# find processes with stdout connected to own stdin
pids=$(echo "$lsof" | awk '$5=="1" && $7=="'"$pipe"'" {print $2}')

# list full command for first process
for p in $pids; do
    ps -p $p -o command= 2>/dev/null && break
done

Derived from https://superuser.com/a/244427